The shelf life of UHT (ultra-heat temperature) milk is as known limited by the natural or microbe-induced enzyme activities of milk. The best known of these problematic enzymes is the plasmin enzyme system that is very heat-stable. During warm storage, they cause changes in the taste, structure, etc.
In addition to the above-mentioned activities, low-lactose and lactose-free milks include side activities, such as proteolytic activities.
In addition to enzymatic changes, the shelf life of milk is also limited by Maillard browning products produced by the Maillard reaction which are especially problematic in lactose-hydrolyzed products. The Maillard browning products cause an undesirable change in the organoleptic properties of UHT milk, such as taste, colour, and structure. Monosaccharides, glucose and galactose, obtained in lactose hydrolysis are more reactive than lactose, which causes stronger Maillard browning reactions. In hydrolyzed milk, the molar content of these reducing monosaccharides is almost double in comparison with that of regular milk lactose.
The reduction of active lysine which is an important amino acid for the nutritional value of a milk-based UHT product continues during the storage at room temperature after the heat treatment. The Maillard reaction and lysine destruction continue during storage.
Heat treatment produces a complex between the β-lactoglobulin and the κ-casein of casein micelle (β-κ-complex). Gelation most probably takes place in two phases: in the first phase, the β-κ-complex detaches from the casein micelle during the storage of UHT milk, and in the second phase, a three-dimensional gel network is formed between the complexes. The precipitation occurring in UHT milk during storage is a result of slow proteolysis. Before gelation, κ-casein-type compounds are formed and the number of non-protein nitrogen compounds increases. The hydrolysis of proteins, i.e. proteolysis, may also promote casein precipitation under the effect of heat. Proteases formed by the natural microbe flora of milk mainly affect the formation of γ- and para-κ-casein, the proportion of which in the total casein is small. In UHT milk, proteases split κ-casein whose stabilising effect in casein micelle is, however, essentially important (Datta, N. and Deeth, C. Age gelation of UHT milk, a review. Food and Bioproducts Processing, 2001; 79(C4): 197-210).
Heat treatments and aseptic packaging methods are known in the field. UHT heat treatment may be either direct (vapour in milk, milk in vapour) or indirect (tube heat exchanger, plate heat exchanger, scraped-surface heat exchanger).
The preparation of low-lactose milk products is generally known. Several processes have been presented for removing lactose from milk. A conventional enzymatic process for the splitting of lactose is generally known in the field, the process comprising the step of adding lactase from fungus or yeast into milk in such a manner that lactose is split into monosaccharides, i.e. glucose and galactose, in over 80%.
Processes for removing lactose from milk raw material are also known, especially by using membrane techniques. Four basic membrane filtration processes are generally used: reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF), and microfiltration (MF). Of these, UF is mainly suitable for separating lactose from milk. Reverse osmosis is generally applied to concentration, ultra- and microfiltration to fractionation, and nanofiltration to both concentration and fractionation. A lactose removal process based on a membrane technique is described in WO publication 00/45643, for instance. A problem with this process, as with membrane techniques in general, is that during ultrafiltration not only lactose is removed from the milk, but also some of the salts that are significant for the taste of milk and milk products prepared thereof. A problem with ultrafiltration is also that it is difficult and expensive to achieve high protein content (over 80% of dry matter).
FI publication 115752 discloses a process in which a milk product is ultrafiltered, nanofiltered, and concentrated by reverse osmosis, after which the salts removed during ultrafiltration are returned to the UF retentate. The residual lactose of the thus obtained low-lactose milk product is hydrolyzed with a lactase enzyme into monosaccharides, whereby an essentially lactose-free milk product is obtained. With this process, lactose is removed from milk without affecting the organoleptic properties of the milk product being prepared.
A chromatographic separation is a process known per se and in industrial use in the sugar industry (examples are the separation of saccharose from molasses and fructose from a glucose-fructose mixture) as well as in the fractionation of whey (U.S. Pat. No. 3,969,337). In the process described in FI publication 78504 for recovering pure lactose from the whey of milk or cheese, a main part of lactose is first crystallised and the liquid of crystallisation purified by heating is fractionated chromatographically.
Lactose can also be specifically separated from milk by chromatography. However, many problems differing from the processing of whey are associated with the processing of milk, such as easy precipitation of casein, maintaining the micellar structure of casein, behaviour of fat, and extremely strict hygiene requirements. For instance EP publication 226035 B1 describes a lactose separation process in which milk is fractionated in such a manner that the lactose fraction is separated and the salts are in the protein fraction or protein-fat fraction. The process is characterised by balancing cation exchange resin by making its cation composition correspond to that of milk, and milk is chromatographed in a column with the balanced cation exchange resin at a temperature of approximately 50 to 80° C. by using water in elution. An advantage of the process is that all compounds essential to taste remain in the milk. However, chromatographic lactose separation is a slow and complex process that cannot directly be applied to conventional dairies without expensive equipment investments. The specific separation of milk lactose described in the patent publication was performed in a laboratory-scale column and the entire separation lasted 28 to 34 minutes. The separation was performed at 65° C. This treatment is not sufficient to inactivate the plasmin enzyme system. The patent publication also disclosed the option of performing the separation at a temperature of 80° C., but the whey proteins of milk then denature significantly. The process is not suitable for the preparation of a UHT milk drink with a long shelf-life.
it is also known to use milk as raw material after lactose removal in the preparation of dairy products. Recent studies have concentrated on membrane filtration of milk and on the use of such milk in the preparation of dairy products, such as cheese, ice cream and yogurt. Common to the known processes for preparing lactose-free products is that prior to ultrafiltration, milk is standardized to a desired fat content and pasteurized by heating it to a temperature of 60 to 90° C. Pasteurization is not sufficient to inactivate the plasmin enzyme system.
In the preparation of UHT milk combined from milk powder, the increase in the temperature of the preheating treatment from 75-80° C. to 90° C. improved the shelf life of UHT milk at room temperature (20 or 30° C., 8 months) (Newstead et al., Int. Dairy J. 16:2006, 573-579). The shelf life was estimated by monitoring the amount of sediment at the bottom of the package. Efficient inhibition of the plasmin-type proteolysis was given as the reason for the improvement in shelf life. In lactose-hydrolyzed milk, the preheating according to the publication or strengthening the UHT treatment, for example, strengthens the Maillard reaction that causes Maillard browning and taste defects and weakens the nutritional value as the amount of available lysine decreases.
Various pre- and post-heating treatments in the preparation of UHT milk are known to improve the shelf life of UHT milk. Driessen reduced the activity of plasmin and improved the shelf life of UHT milk by pre-heating milk at 55° C. for 60 minutes before high heat treatment. Milk proteolysis, bitterness and the development of transparency lessened and no gelatination occurred when milk was preserved at 20° C. for 11 weeks (Datta and Deeth, 2001). Kocak and Zadow found that the treatment of lactose-hydrolyzed milk after the UHT process at 55° C. for 40 to 60 minutes about doubled the shelf life time of the product (Aust J Dairy Technol 1989; 44(1): 37-40).
WO publication 2004/019693 describes a process for separating milk into individual components with membrane techniques and combining these components into milk products, such as ice cream, set-type or stirred yogurt and milk drink. The reconstituted milk raw material may be skim milk, low-fat milk, full-fat milk, lactose-free milk, concentrated milk, milk powder, organic milk or a combination of these.
Combining membrane methods and ion exchange for the preparation of a milk product, such as low-calcium milk powder, is known. WO publication 01/41579 describes a process that uses strong cation exchange resin preferably at a temperature of 4 to 12° C. The process can also be performed at 50° C. However, the 2.5-hour ion exchange treatment described in the patent publication is not sufficient to inactivate the plasmin enzyme system. In the process, the improvement in heat stability is based on reducing the calcium content with ion exchange.
A general problem with the known lactose removal processes and high heat treatments is a change in the organoleptic properties of milk or milk products made thereof. When making completely lactose-free products, in which the residual lactose content requirement is less than 0.01%, it is necessary to add more lactase enzyme than in low-lactose products. The taste of lactase-hydrolyzed milk products is not pure, but often contains a musty, chemical- or medicine-like off-taste, especially towards the end of the sales time. Known processes are also characterized in that products requiring a long shelf life time at room temperature (e.g. UHT products) show structural problems (precipitation, sedimentation), and in lactose-hydrolyzed products in particular, the Maillard reaction causes Maillard browning and changes in taste. A reduction of the nutritional value of the product is also associated with Maillard browning.
Trade and consumers require longer and longer sales times. The shelf life at room temperature of lactose-free and low-lactose UHT milk products made by conventional techniques is limited, typically three months. It is thus desirable to provide natural processes with which the shelf life of the organoleptic properties, such as taste properties, of the products, and their structure can be improved, which would then also extend the sales time.
UHT milk is milk that has been treated continuously at a high temperature for a short time and that has thereafter been immediately packaged aseptically. The heat treatment must be such that UHT milk passes the shelf life test and gives a positive result in the turbidity test (IDF Doc 2/1970, ref. IDF Annual Bulletin Part V, IDF Monograph on UHT milk, 1972) (in other words, some of the whey proteins remain undenatured). One part of the preservability test is a storage test according to which UHT milk must remain unchanged in its package at a temperature of 30±1° C. for 14 days. Low-lactose (e.g. Hyla®) and lactose-free milk already brown clearly during this type of storage.
A process for the preparation of low-lactose or lactose-free milk products (milks, milk drinks, whey drinks, concentrates) that are completely flawless in taste and have good shelf life without any extra costs has now been unexpectedly invented. The process of the invention also makes it possible to ensure improved shelf life for the product, enabling a longer than usual sales time, whereby structural defects, such as precipitation, decrease in nutritional value, risks related to microbiological problems, and the Maillard reaction, are minimized.